[riot-devel] periph/rtt intent

Michael Andersen michael at steelcode.com
Thu Jul 13 22:29:27 CEST 2017

A brief aside, as far as I recall I had to make the timer interface also
integrate the RTT to get xtimer to use the RTT on SAMR21. It was very ugly,
which is why I did not upstream it.

On Thu, Jul 13, 2017 at 1:18 PM, Hauke Petersen <hauke.petersen at fu-berlin.de
> wrote:

> Hej!
> For the motivation of creating the RTT interface it is needed to once more
> recap the main motivation of the peripheral drivers: the idea is/was to
> introduce unified APIs that give direct access to selected MCU peripherals
> with the least possible overhead, mapping each type of peripheral to
> exactly one API. So it was never intended for the periph interfaces to care
> of multiplexing between different hardware units of the same/simliar type,
> but this was always intended to be done by higher layer modules (as e.g.
> the xtimer, stdio, etc).
> So the original reason for creating the RTT interface was actually
> historically driven by the stm32's and the sam3, where especially the stm32
> have three different hardware units (timer, rtt, rtc), all with some
> different characteristics. So we decided back then, to create different
> periph APIs for each of these units.
> Now from todays point of view (and we have learned a lot in the last 3
> years...), I think what is nagging is most conceptionally is that the
> periph API and the way its used does not allow us to concurrently map
> different devices into the same API (e.g. timer vs lptimer, uart vs.
> lpuart, rtt vs tim, etc). The only way this is currently possible is to use
> (very) ugly if-else constructs in the periph drivers imlementation, that
> switch between two (or more?) completely different code paths. This is not
> only ugly, hard to maintain, and imposes run-time penalties, but this also
> breaks with the original intention of the periph API to provide slim and
> direct access to a single selected type of peripheral.
> So back to the RTT topic: considering what one would expect from the API,
> there is indeed no good reason for having a separation between periph/timer
> and periph/rtt. Both provide the same type of functionality and the
> periph/timer interface could simply also be implemented for rtt hardware
> units. But to do this cleanly, we would need to solve the (internal)
> multiplexing issue for using different typed peripherals under the same
> API. There was some prior work done by some people in the community
> (kaspar, johann, ...) to play around with function pointer based approaches
> which would solve this issue natively, but so far no solution yielded
> satisfying results and could compete in terms of efficiency.
> So bottom line: functional the RTT interface does only make limited sense,
> but structurally given by the motivation of the periph interfaces there is
> a reason for having it. I guess the problem we need to solve is more about
> internally multiplexing peripheral types to the same API...
> Cheers and thanks for bringing this up,
> Hauke
> On 13.07.2017 21:32, Joakim Nohlgård wrote:
> Dear developers,
> The intent of the RTT API seem to be rather unclear. My interpretation was
> that the RTT API is used for long time, low precision timers that can be
> used to sleep the system for minutes, hours or days. The precision is
> expected to be 1 second, the same as for the RTC API, but without the
> calendar struct representation. The reason for using the RTT API is to let
> it represent the current wall clock time, for tagging collected data etc,
> while the xtimer (and periph timer) is used for delays and internal
> precision timing of events. The RTT alarm is used to schedule wake ups or
> events which should happen at a specific time of day, for example taking a
> measurement every day at 2:00.
> There seem to be a different interpretation as well: the RTT API is used
> for any hardware timers which keep running in low power modes.
> This split is seen in the board configurations, where some boards list the
> RTT as a 32768 Hz timer, others list it as a 1 Hz timer. This has already
> begun causing problems among the API users, see the lwmac implementation
> for example, which only works if the RTT is 32768 Hz.
> My interpretation was initially based on a limitation in the RTT (called
> RTC in NXP documents) on Kinetis hardware which only allow integer seconds
> for the alarm, while a different hardware module can be used for 1/32768
> second precision, but that timer wraps around after 2 seconds (2**16
> ticks).
> We need to clarify the documentation and align the implementations to
> match each other, or the RTT API will be useless as an abstraction layer,
> since its interpretation will be platform dependent.
> What are your opinions on the RTT API, or the timer APIs in general?
> Best regards,
> Joakim Nohlgård
> RIOT developer
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